This invention relates to a system for processing signals from an overheat monitor for a gas-cooled dynamoelectric machine. In particular the invention concerns an electronic control which automatically determines the validity of an apparent overheating signal and initiates a reduction in machine load or a monitor malfunction signal.
Overheating in stator cores of gas-cooled dynamoelectric machines, such as that resulting from foreign object damage to stator teeth allowing electrical contact between core laminations and thus resistance heating, can rapidly cause local melting of core laminations, leading to costly machine repairs and outages. Other overheating, for example, to other iron or copper parts of the stator can cause costly and increasing damage. To minimize damage, it is very desirable to detect overheating in its earliest stages so that load carried by the machine can be reduced and appropriate corrective action taken at minimal cost and outage time.
Systems for detection of machine overheating are disclosed in several U.S. Pat. Nos. assigned to the assignee of the present invention, i.e., 3,427,880 to Grobel et al; 3,573,460 to Skala; 3,702,561 and 3,916,671 to Carson et al, the disclosures of which are incorporated herein by reference thereto. The systems described therein are based primarily on the principle that at onset of overheating in a dynamoelectric machine, decomposition of materials such as the coatings normally applied to certain machine parts (or intentionally applied for the express purpose of detecting overheating) yields pyrolysate products including submicron particles, which become entrained in the machine gas coolant. U.S. Pat. No. 3,573,460 to Skala discloses an ion chamber detector in which gas is ionized and electrical current resulting from the ionized gas is measured to determine whether submicron particles are present; the presence of submicron particles causes a measurable decrease in current because some ions collide with and attach themselves to the particles, decreasing the mobility of these ions and the likelihood of their collection and contribution to ion current.
This ion chamber detector is applied to an overheat monitor in U.S. Pat. No. 3,427,880 to Grobel et al according to which disclosure a portion of the gas coolant from a dynamoelectric machine is passed through the ion chamber detector and the resulting current monitored for presence of pyrolysates.
Further refinements to overheat detectors are disclosed in U.S. Pat. Nos. 3,702,561 and 3,916,671 to Carson et al and U.S. patent application Ser. No. 783,772 (Barton et al) filed Apr. 1, 1977. In the disclosure of U.S. Pat. No. 3,702,561 a submicron particle filter is included to remove particles from the gas coolant and verify the overheat signals by detection of an increase in ion current level, and a coated filament is provided to check the performance of the detector by producing pyrolysate particles to simulate overheating. U.S. Pat. No. 3,916,671 discloses the addition of gas chromatographic apparatus to the monitor system for identifying specific pyrolysate materials and thus their source within the machine. U.S. patent application Ser. No. 783,772 discloses the use of two ion chamber detectors, one of which can be heated to gasify any oil mist particles present, thus providing means for differentiation between a detector output due to machine overheating and a detector output due to oil mist particles entrained in the gas coolant.
While the foregoing prior art inventions represent significant advances in early detection of local overheating in a dynamoelectric machine, their use in practice has not been as effective as may be desired because of the lengthy period of time required to fully respond to a machine overheat and because the judgment and actions of each individual machine operator may lead to different and sometimes inappropriate responses to apparent overheating. Thus in certain circumstances and installations, as much as five minutes may elapse after onset of overheating before corrective action is completed; during this time there is first cognizance of apparent overheating, followed by dispatch of personnel to the location of the overheat detector (which may be remote from the main control room), then activation by these personnel of a verification system such as a particle filter, and finally, after overheating is validated, a reduction in machine load to a level at which overheating ceases or the excitation system and main line breaker of the machine may be tripped. During the several minutes which elapse prior to reduction of load to safe operating levels serious damage may occur to the machine such as melting of the core. Alternatively, if load reduction or runback is initiated prior to validation of overheating, unnecessary loss of power generation and inefficient use of fuel may occur if, after a lengthy verification sequence, the drop in signal is proved spurious, e.g., caused by ion chamber detector current leakages, calibration errors, or other malfunctions associated with peripheral recording and amplification equipment of the monitor.
Accordingly, it is a general object of the invention to provide a system to automatically process signals from an overheat monitor for a gas-cooled dynamoelectric machine of the type used to generate electrical current in response to a rotating field.
It is another object of the invention to provide a system to rapidly determine the validity of an apparent overheat signal as indicated by a drop in output signal from an overheat monitor.
An additional object of the invention is to eliminate the subjective evaluation of apparent overheat signals and provide therefor an electronic control system of design having high operational reliability.
A further object of the invention is to provide a system which will automatically initiate reduction in load of a dynamoelectric machine, trip the machine, or indicate a malfunction of the overheat monitor in conjunction with determination of overheat signal validity.